Actually, it still uses chemical propellant according to the Aliens: Colonial Marine Tech Manual. I don't have my copy right now, but IIRC it says the Pulse Rifle's rounds are 10x24 caseless with a muzzle velocity of 840 m/s.
There's quite a bit of cool stuff in the Tech Manual like hyper velocity fire-and-forget AT weapons, info on the class of ship the Sulacco belongs to, a Heavy Tank with a PD laser and 115mm main gun that can fire at up to 60 RPM, etc. If you want I can PM you quotes once I have my copy back.

George Krstic @GeorgeKrstic The Council has been summoned! Meeting with @TitmouseInc @chrisprynoski tonight to talk #bringbackmotorcity & #bringbackmegasxlr stay tuned! 8:04 p.m. Mon, Apr 29
http://gamepodunk.ipbhost.com/uploads/gallery/album_6/gallery_5_6_15133.png
Spread the word across the internet my fellow ASVSers! This great show may yet return from the grave.

Barrier drops? Evil cloud with eyes attacks? Oh come on now, did the writers have a fucking time machine or something?
So THAT'S what happened to my upgraded laser rifle from FO:NV!
Changelings! Changelings everywhere! Can’t stand them.
*head explodes like in the GIF*

Wow. 5 miles long and only 2.5 million tons? Did they buy Titanium-A from the UNSC or something?
Still, JMS did do a better job than Halo:
Beta probe cycled back. The mass was still there and as
solid as before. It was the largest reading Ensign Lovell had
ever seen: twenty thousand tons. That couldn't be a Covenant
ship--they didn't get that big. - Page 137, TFoR
It turned out to be 4 Covenant warships:
1x Carrier (1455 meters long)
1x Destroyer (1500 meters long)
2x Frigates (1000 meters long)